Lead Essay Research Paper Lead symbol Pb

Lead Essay, Research Paper

Lead, symbol Pb, dense, bluish-gray metallic element that was one of the first known metals. The atomic number of lead is 82; the element is in group 14 (or IVa) of the periodic table. Lead was mentioned in the Old Testament. It was used by the Romans for making water pipes, soldered with an alloy of lead and tin. Few metals have been used in more different ways than lead. Lead ornaments and coins have been in use since ancient times. In the Middle Ages strips of lead called cames were first used to assemble the pieces in stained-glass windows. In the United States about two thirds of all the lead produced is used in electric storage batteries of the kind found in automobiles. Large amounts of lead are used as protective coverings for electrical cables, and there are lead compounds in paints and pigments. The use of lead in paints has been decreasing because lead is poisonous, and many children have been poisoned by eating flakes of dried paint containing lead. Lead bullets and shot made gunpowder effective in firearms. Lead is the main ingredient of solder, used for joining pieces of metal.

Metallic lead is a soft, malleable, ductile metal. Lead is the heaviest and the softest of the common metals. When gently heated it can be forced through annular holes or dies. It has low tensile strength and is a poor conductor of electricity. A freshly cut surface has a bright silvery luster, which quickly turns to the dull, bluish-gray color characteristic of the metal. Lead melts at 328. C (662. F), boils at 1740. C (3164. F), and has a specific gravity of 11.34; the atomic weight of lead is 207.20. Because it resists attack by air and water as well as by many strong chemicals, it can be used to protect electrical cables, or to line large vessels in which chemical processes are carried out. Screens to protect people from X rays, gamma rays, and radioactive materials are made from lead because it absorbs radiation. For this reason lead is used as a protective shield around nuclear reactors and particle accelerators.

Safety plugs and engine bearings are only two of the many products made of alloys that contain lead. Some lead alloys melt so easily that they change to liquid when held in the hand. Useful lead compounds include lead monoxide, or litharge.

This substance makes up a large part of the brilliant, lustrous glass called flint, or crystal, that is shaped into vases, bowls, drinking glasses, and lenses. Another oxide of lead called minium, or red lead, is used in paints that protect iron and steel from rusting. Basic lead carbonate (white) and lead chromate (yellow) are pigments in paints. Lead azide, easily exploded by an electrically heated wire, is used in blasting caps to set off other explosives.

The most abundant ore, or source, of lead is the mineral galena, or lead sulfide. Its chemical formula is PbS. Other important ores are cerussite (lead carbonate, PbCO3) and anglesite (lead sulfate, PbSO4). Most lead ores contain zinc, and many also contain gold, silver, or other metals. Lead is soluble in nitric acid but is little affected by sulfuric or hydrochloric acids at room temperature. In the presence of air, it slowly reacts with water to form lead hydroxide, which is slightly soluble. Lead is toxic when taken internally; although ordinary water usually contains salts that form a coating on pipes, inhibiting the formation of soluble lead hydroxide, pipes used for carrying drinking water should not contain lead.

Lead occurs naturally in eight isotopic forms, of which four are stable and four radioactive. The stable isotopes, lead-206, lead-207, and lead-208, are, respectively, the end products of the uranium, actinium, and thorium series of radioactive decay; lead-204, also stable, has no natural radioactive precursors.

Lead is widely distributed all over the world in the form of its sulfide, the ore galena. Lead ranks about 36th in natural abundance among elements in the earth’s crust. Ores of secondary importance are cerussite and anglesite. The principal method of extracting lead from galena is to roast the ore-that is, convert it to the oxide, and reduce the oxide with coke in a blast furnace. Another method is to roast the ore in a reverberatory furnace until part of the lead sulfide is converted to lead oxide and lead sulfate. The air supply to the furnace is then cut off and the temperature raised; then the original lead sulfide combines with the lead sulfate and lead oxide to form metallic lead and sulfur dioxide.

The ore is first pulverized, and the metal-bearing material is separated from the rock by the flotation process; that is, it is mixed with water and certain oils and chemicals, and air is blown in from the bottom of the mixing container. The lead-bearing particles are wetted by the oil and float to the surface attached to air bubbles. The waste, called gangue, is wetted by water; it sinks to the bottom and is discarded.

The concentrated ore is roasted in the presence of air to change lead sulfide to lead oxide. In the process, sulfur escapes in the form of the gas sulfur dioxide, which can be recovered and made into sulfuric acid. The crude lead oxide is smelted in a blast furnace or open-hearth furnace with coke and a flux of silica or lime. The lead metal settles to the bottom, dissolving and carrying with it any gold or silver that was present in the origin